What are or What is@tchrist: I take your point, but what about "Who is your mother and your best friend?" Your mother and father have to be two different people, so "is" is obviously incorrect, but a cause and a consequence can be the same thing, so either "is" or "are" could be correct, depending on the specific context.

Aug31

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What are or What isIf you only have one of each, you need to use "is". If not (i.e. if there are multiple causes and/or consequences), you need to use "are".